The blockage of a drainage line on a thin layer vaporiser (separation of dimethoate by distillation of the azeotrope dichloromethane-water mixture in continuous operation) leading to the discharge pumps and the malfunction of the radiometer measuring head caused the dimethoate to flow back into the vaporiser facility. This led to overheating and spontaneous thermal decomposition of the dimethoate. A rupture disk burst and 25 kg of dimethyl decomposition products, as well as 3,500 litres of organic sulfur compounds were sprayed 15 m into the air. The foul-smelling cloud drifted eastwardly and 10 individuals were hospitalised for discomfort and nausea after inhaling mercaptan. The entire unit was shut down and the sector was secured.

The accident was caused by an error of the subcontracting company responsible for the replacement of the vaporiser: the transport protection of the latter were not removed before installation. In addition, the radiometric probe was not suitable for the installation. The operator will establish procedures and guidelines for the implementation and monitoring of the works (checks at the reception …).